This invention relates to the field of optical engineering, and particularly to the design of efficient, cost-effective solar energy collectors. Sunlight falling on the earth represents a source of radiant energy, even though at any location on the earth it is not continuous, being cyclically interrupted in accordance with the diurnal rotation of the earth.
Radiant energy from the sun reaches the unclouded earth at a maximum flux density of about 1000 watts per square meter of surface impinged perpendicularly. Solar energy at this natural intensity is suitable for numerous purposes, including lighting and photosynthesis, but is not in a form readily usable for a large number of other applications.
Much thought has been devoted to developing apparatus for converting solar radiant energy to other forms of energy which are more generally useful. Devices for performing this function are known as solar collectors, and simple collectors may do no more than accept radiant energy at its natural intensity and enable it to fall on absorbent material, thereby converting it to thermal energy which is then used to raise the temperature of a suitable, usually liquid, heat transport medium. Flow of the medium transports the heat energy to a more or less distant point of use. A typical apparatus of this sort is known as a flat-plate collector. The problems caused by periods of darkness intervening between periods of insolation, and by the frequent presence of cloud cover, are common to all solar energy collection systems, but are not addressed as a part of the present invention and will not be considered further here.
For many applications, the flux density of solar radiant energy is inadequate: efficient power generation with superheated steam is one example. To meet the demands of such applications, solar collectors have been designed which increase the effective flux density by concentrating the energy incident at an entrance aperture of a first area, so that it is directed to an exit aperture of considerably smaller area where an energy absorber or other useful receiver is located. For example, if the energy at normal intensity reaching an entrance aperture thirty centimeters wide is concentrated to reach an exit aperture three centimeters wide and of the same length, the energy at the exit aperture has a flux density ten times as great as that at the entrance aperture, assuming no loss of energy during concentration. In this discussion it is to be understood that the effective size of an aperture is not measured directly by its physical dimensions, but rather by the components of those dimensions normal to the direction of incident energy, that is, to the "sun line".
The direction of the sun from any point on the earth's surface is not constant. It varies both in altitude and in azimuth from sunrise to sunset, and it also varies seasonally. At 40.degree. north or south latitude the sun's altitude varies from 0.degree. at sunrise to a maximum angle of 73.4.degree. at noon of the summer solstice.
Any square meter of collector surface fixed to the earth, even if perpendicular to the sun line at a particular time of a particular day, is for most daylight hours not perpendicular to that line, and thus effectively represents less than a square meter of entrance aperture. With respect to this problem, solar energy collectors may be divided into two categories defined broadly as tracking collectors and non-tracking collectors, both categories being capable of design to concentrate the radiant flux density.
A tracking collector is one which is mounted for movement with respect to the earth's surface, and in which the collector is moved by a diurnal tracking mechanism to keep it pointed directly at the sun as it apparently moves through the heavens, so that the entire aperture is always perpendicular to the sun line from morning to night, and hence is always of maximum effective area. The requirement for tracking movement of the collector obviously places practical limitations on the area of the collector.
A non-tracking collector is one which is fixed to the earth, and is therefore subject to diurnal change in effective entrance aperture. It is not, however, subject to practical limitations as to physical size, which therefore becomes limited only by the area available for use and the cost of materials and labor. A typical non-tracking collector comprises one or more sets of elongated reflectors each concentrating energy from the sun on an elongated narrow receiver such as a water pipe. The reflectors and receivers extend east and west, and may be constructed in banks oriented at an angle which deviates from the horizontal in accordance with the latitude of the location.
In a tracking collector the upper limit of concentration is accomplished when all the radiant energy received at the entrance aperture is focused or imaged at the exit aperture, where a suitable energy receiver is positioned. Application, to non-tracking collectors, of the optical principles of focusing of energy does not give promise of sufficiently great multiplication of the flux density from entrance to exit apertures.
It is possible, however, to design a solar collector that concentrates the rays of the sun but does not focus them. Such a device is called a non-imaging collector, and to forego imaging is to gain a degree of design freedom that can be put to useful ends.
Non-imaging, non-tracking collectors are not without problems, however. First, a collector of this sort for full coverage must be designed with an acceptance angle at least as great as the range of the sun's altitude angle, or some of the radiant energy will escape collection. Second, reexit of rays which have entered the collector must be prevented, as such rays are lost to the collector and reduce the flux density at the exit aperture.